The present invention relates to a pattern forming method, and more particularly to a pattern forming method capable of easily forming a resist pattern having a good cross-sectional shape.
Attempts to realize a higher integration level of (Ultra Large Scale Integrated Circuits (ULSI) have been made quickly corresponding to four times the initial integration density within a period of three years. Mass production of a 4 Mbit DRAM and a trial manufacture of a 16 Mbit DRAM have already been conducted. The minimum size required in the production of a ULSI has changed from 0.8 .mu.m to 0.5 .mu.m, a further to 0.5 .mu.m or less, and thus the ULSI structure is becoming more and more minute. For matching such trend to a smaller structure, studies are being made to improve the resolution in various lithography techniques.
In photolithography, which is presently a major lithography technique, a pattern forming method which utilizes a shorter wavelength of light than the light used heretofore is now under study for the improvement of resolution. However, the shorter the wavelength of light, the larger the amount of light absorbed in a resist film, and hence a pattern shape is more apt to be deteriorated. To solve this problem, it has been considered necessary to use a resist material of a high light transmittance. In electron beam lithography which will be a main lithography technique in the next generation, a resist material of high sensitivity is desired for the improvement o productivity.
Recently, new resist materials of high performance which utilize a chemical amplification reaction, or a catalytic reaction, have been attracting attention of many concerns, as shown, for example, in J. Vac. Sci. Technol B6(1), Jan/Feb '88, pp. 319-322, "Nanolithography with an acid catalyzed resist" and ibid., pp. 379-383, "Characterization of a high-resolution novolak based negative electron-beam resist with 4 .mu.C/cm.sup.2 sensitivity."
In resist material utilizing a chemical amplification reaction a material is included which produces a catalytic substance upon exposure of an energy ray thereto. This resist material is characteristic in that the intermediate substance created by the exposure of an energy ray serves as a catalyst in the reaction of resist during a subsequent treatment such as heat treatment, thereby permitting the reaction to proceed efficiently. Therefore, the transmissivity and sensitivity can be enhanced as compared with conventional resist materials.
However, it turned out in an experiment conducted by the present inventors that when the above chemically amplified resist was applied onto a film of spin on glass (the spin on glass generally indicates a compound having such a siloxane structure as illustrated in FIG. 8 wherein Z.sub.1 and Z.sub.2 are each independently an alkyl, alkoxyl, acetoxyl or hydroxyl group or hydrogen or a siloxane compound and n represents a mean condensation number of the spin on glass), followed by exposure and development in a conventional manner, an abnormal condition occurred in the cross-sectional shape of the resist pattern. This profile abnormality in the section of the resist film is presumed to be a phenomenon which occurs due to a decrease in the amount of the catalytic substance in the resist material in a position near the base film and the resulting inhomogeneous distribution of the catalytic substance in the thickness direction of the resist film. Particularly, in a negative type resist wherein an energy ray-exposed portion remains after development, an abnormal undercut is formed in the bottom of the pattern, resulting in falling or delamination of the resist pattern. This is a serious problem which must be solved. A more detailed explanation of this phenomenon will be made below with reference to FIG. 2a and 2b.
In the case of forming a resist pattern by a three-layer resist method using an antireflective film 204 of an organic compound as a bottom layer, a spin on glass film 203 as a middle layer and a chemical amplified resist film 202 of, for example, a negative type as a top layer, a catalytic substance 205 is formed in a portion exposed with energy ray 201 and a latent image of pattern is formed. However, a drop-out portion 206 of the catalytic substance 205 is formed in the resist film 202 because the amount of the catalytic substance in the resist film 202 is decreased by the spin on glass film 203, resulting in that the distribution of the catalytic substance 205 in the interior of the resist film 202 becomes inhomogeneous as shown in FIG. 2a. Since the catalytic substance 205 acts to induce a crosslinking reaction, the same reaction does not occur in the catalytic substance dropped-out portion so that the cross-sectional shape of the resist pattern 202 formed by development becomes abnormal as shown in FIG. 2b. Consequently, the kind of base materials capable of processing into a predetermined shape using a chemical amplified resist is markedly restricted and it becomes difficult to use this resist in the manufacture of a ULSI circuit.